1. Field of the Invention
This invention relates to an automatic focus adjusting device adapted for use in a video camera or the like.
2. Description of the Related Art
The advancement of video apparatuses such as video cameras, etc., has been conspicuous during recent years. The video apparatuses are now generally arranged to have versatile functions including automatic focus control, automatic iris control and zooming functions, etc., and also to have improved operability in every function.
with regard to automatic focus adjusting devices for performing the automatic focus control, a method of adjusting focus by detecting the sharpness of a picture from a video signal obtained by photo-electrically converting an object image at an image sensor and then by controlling the position of a focusing lens in such a way as to make the sharpness of the picture maximum has been becoming a main stream.
To evaluate the sharpness of the picture, use is generally made of the level of a high-frequency component of the video signal extracted through a band-pass filter of a certain frequency band, i.e., a focus voltage. In shooting an ordinary object, the level of the high-frequency component (focus voltage) increases accordingly as the state of focus becomes sharper. Thus, an in-focus state is considered to be attained at a point where that level reaches a maximum level.
However, the conventional automatic focus adjusting device has prevented the following problem. In moving a focusing lens by a predetermined amount of movement at a time, when the direction of moving the focusing lens is inverted, a play between a rack and a shaft which are arranged to drive the focusing lens sometimes tends to prevent the focusing lens from being moved by an amount of movement proportional to the amount of rotation of a motor, as shown by a line "c" in FIG. 8, even if the motor is rotated by a certain amount. As a result, there arises an error in the position of the focusing lens. The adverse effect on the focus caused by the error of the position of the focusing lens is greatly increasing as the image pickup plane of the image sensor (CCD) is becoming smaller these days. The abovementioned play makes it hardly possible to secure an accurate amount of movement of the focusing lens, so that a precise AF (automatic focusing) evaluation value can not be obtained. The inaccurate evaluation value then might result in misjudgment about the direction in which to move the focusing lens.